Glass fibers and glass fiber strands have been used before in the art to produce various types of glass fiber mats for use as reinforcement material. The basic principles of mat-making are well known in the art and are fully described in the book entitled "The Manufacturing Technology of Continuous Glass Fibers" by K. L. Lowenstein, published by the Elsevier Publishing Company, 1973 at pages 234 to 251. Typical processes for making mats of continuous fiber glass strands are also described in U.S. Pat. Nos. 3,883,333 (Ackley) and 4,158,557 (Drummond).
Typically, the mats formed by these processes are needled in order to provide sufficient mechanical integrity to the them. In the needling operation, rapidly reciprocating barbed needles are used to cause the individual glass strands which make up the mat to become entangled with one another thus resulting in a mat that can be subsequently handled and processed. The needling operation typically used is described in U.S. Pat. Nos. 3,713,962 (Ackley), 4,277,531 (Picone) and 4,404,717 (Neubauer, et al.) Mechanical integrity can also be imparted to the mat by depositing a resin on its surface and curing or melting the resin so that individual strands are bonded together.
A particular utility for glass fiber mats is in the reinforcement of resinous or polymeric materials. The presence of a glass fiber mat provides increased strength over that of the unreinforced material. Usually, the mat and molten resin are processed together to form a thermosetting or thermoplastic laminate. Thermoplastic laminates are particularly attractive for use in the aircraft, marine, and automotive industries since they may be reheated into a semi-molten state and then stamped into panels of various shapes such as doors, fenders, bumpers and the like. It is of the utmost importance, however, that the glass mat used to make the laminate be as uniform as possible in both its thickness and fiber density as measured in units of ounces per square foot. If a non-uniform mat is used for reinforcement purposes, the reinforced products produced therefrom may have a substantial variation in their strength since some areas may be weaker due to the lack of glass fiber reinforcement while others may be stronger. Even more important is the need to insure that the glass reinforcement flows or moves freely within the thermoplastic laminate during the stamping operation in order to produce uniform strength properties in the final component.
In the production of continuous strand mats by the aforementioned patented processes, a plurality of strand feeders are positioned above a moving belt or conveyor. The conveyor is typically a flexible stainless steel chain. The strand feeders are reciprocated back and forth above the conveyor parallel to one another and in a direction generally across the width of the moving conveyor. Strands of multiple glass fiber filaments are fed to the feeders from a suitable supply source such as a plurality of previously made forming packages held in a support rack generally known in the art as a creel. Each feeder apparatus provides the pulling force necessary to advance the strand from the supply source and deposit it on the surface of the moving conveyor. In a typical production environment, as many as 12 to 16 such strand feeders have been used simultaneously with one another so as to produce a mat with as uniform a density distribution as possible.
It is also well known in the art that the feeder can act as an attenuator to attenuate glass fibers directly from a glass fiber-forming bushing and eventually deposit the strands so formed directly onto a conveyor as described by Lowenstein, supra at pages 248 to 251 and further illustrated in U.S. Pat. Nos. 3,883,333 (Ackley) and 4,158,557 (Drummond).
An example of a simple traversing mechanism is a feeder mounted on a track where the feeder is caused to reciprocate back and forth by an electric motor capable of reversing directions. The equipment used within this type of configuration has inherent limitations on its mechanical durability. First, the feeders are quite heavy, usually weighing between 30 to 50 pounds. When this heavy apparatus is traversed across the width of the conveyor, the traverse speed is limited due to the momentum of the moving feeder and the impact forces which must somehow be overcome or absorbed upon each reversal of direction. This limitation on the speed at which the feeder may traverse across the width of the conveyor may also limit the rate of mat production. Secondly, this constant reciprocating motion of the feeders causes vibration to occur and this can result in a great deal of wear on the feeder mechanisms and their guides which may eventually lead to mechanical failure.
In U.S. Pat. No. 3,915,681 (Ackley), a reduction in the vibration normally associated with the reversal of a feeder was accomplished by the use of a traversing system in which a feeder was caused to reciprocate back and forth along a track. The feeder was advanced by a continuous chain driven by a motor. The chain had affixed to it an extended member or pin which engaged a slot milled into the carriage of the feeder. The slot was positioned so that its length was parallel to the direction of motion of the chain and had a length substantially greater than the diameter of the pin. Thus, the feeder was caused to reciprocate by the continuous motion of the chain since, as the feeder traveled in one direction, the pin exerted the force necessary to advance the feeder by pressing against the periphery of the slot. When the feeder reversed its direction, the pin slid until it contacted the opposite periphery of the slot at which point motion of the feeder was reversed. When the feeder approached the termination point of its reciprocating stroke, it contacted a shock absorber which decelerated it and absorbed the impact due to the change in momentum. Later, as an improvement on the basic design, these shock absorber members were replaced with gas pistons and a reservoir capable of storing the absorbed energy was used to help accelerate the feeder in the opposite direction (See U.S. Pat. No. 4,340,406 (Neubauer, et al.)).
Although such designs were successful in reducing some of the vibration associated with the reciprocation of the feeders, the pin and slot arrangements introduced additional mechanical components that could fail and cause an interruption in the mat-making process. Also, the shock absorbers and gas pistons were mechanical devices inherently incapable of precise and repeatable acceleration and deceleration rates.
A second problem with the systems taught by the prior art was the inconsistency of the mat produced. In the deceleration/acceleration cycle of the feeders, more glass fibers tended to accumulate on the surface of the conveyor near the terminal end of each traverse stroke thus forming a mat tending to be thicker near its edges than in the more central portions thereof.
The reason for the buildup of glass fibers near the mat edges was because that each time the feeder reversed its direction, it was locally resident for a greater duration of time over those portions of the mat where the deceleration/acceleration cycle occurred, i.e., the edges, than it was anywhere else. As long as the feeder was paying out glass strand at a constant rate during the entire duration of the turnaround cycle, then the edges of the mat could do nothing but accumulate a greater thickness of glass than was present in the interior.
In order to produce a finished mat having a more uniform density, it was often necessary to trim the mat as it left the conveyor. This reduced the efficiency of the process by a significant degree since material lost due to trimming was wasted.
Thus, despite the advances made by the prior art, there still exists a need to (1) more rapidly reverse the feeder apparatus during its turnaround cycle, (2) minimize the mechanical vibration associated with a rapid turnaround of the feeder apparatus, and (3) better control mat uniformity and density.
As will now become evident from the remainder of the disclosure, an improved mat making method is provided which satisfies these needs.